How to fix NullPointerException in breath first search in java?

Question

I try to construct a breath first search relationship(BFS) by using a txt file. It works perfectly when the BFS size is small(eg. 7nodes), but NullPointerException appears when the BFS size is big(eg. 20000nodes). I've already read some posts about NullPointerException, but I still have no idea on how to solve it.

The exception message:

Exception in thread "main" java.lang.NullPointerException
        at GraphAdjacentList.setEdge(GraphAdjacentList.java:16)    
        at GraphAdjacentListDemo.main(GraphAdjacentDemo.java:52)

There are 7 nodes, then each line shows the relationship of two nodes. The format of the txt file:

7
1 2
1 3
1 4
4 5
4 6
5 7

My code:

import java.util.*;
import java.io.*;

public class GraphAdjacentListDemo{
/* to build an adjacentList */
final private Map<Integer, List<Integer>> Adjacent_List;
public GraphAdjacentListDemo(int numberOfVertices){
    Adjacent_List = new HashMap<Integer,List<Integer>>();
    for(int i=1;i<=numberOfVertices;i++){
        Adjacent_List.put(i,new LinkedList<Integer>());
    }
}

public void setEdge(int source,int destination){
    List<Integer> sList = Adjacent_List.get(source);
    sList.add(destination);
    List<Integer> dList = Adjacent_List.get(destination);
    dList.add(source);
}

public List<Integer> getEdge(int source){
    return Adjacent_List.get(source);
}

public static void main(String args[]) throws Exception{
    /* to import a node source file */
    File f = new File("/Users/apple/Desktop/Graph_Sample.txt");
    List<Integer> source = new ArrayList<Integer>();
    int count=0;
    Scanner sc = new Scanner(f);


    while(sc.hasNextInt()){
        int i=0;
        i = sc.nextInt();   
        source.add(i);
        count++;
        }
        sc.close();

    /* to store nodes into source_array and destination_array */
    int time=(count-1)/2;
    List<Integer> sourceList = new ArrayList<Integer>();
    List<Integer> destinationList = new ArrayList<Integer>();
    for(int i=1;i<=time;i++){
        sourceList.add(source.get(1+2*(i-1)));
        destinationList.add(source.get(2+2*(i-1)));
    }

    GraphAdjacentList adjacentList = new GraphAdjacentList(source.get(0));
    for(int i=0;i<sourceList.size();i++){
        adjacentList.setEdge(sourceList.get(i),destinationList.get(i));             
    }

    System.out.println("The graph is:\n");
    try{
    for(int i=1;i<=source.get(0);i++){
        System.out.print(i+"-->");
        List<Integer> edgeList = adjacentList.getEdge(i);
        for(int j=1;;j++){
            if(j != edgeList.size()){
                System.out.print(edgeList.get(j-1)+"-->");
            }else{
                System.out.print(edgeList.get(j-1));
                break;
            }
        }
        System.out.println();
        }
    }catch(InputMismatchException inputMismatch){}


}

}